Port stack formatter

Author: som-snytt

compiler/src/dotty/tools/dotc/util/StackTraceOps.scala

@@ -0,0 +1,76 @@
+/*
+ * Scala (https://www.scala-lang.org)
+ *
+ * Copyright EPFL and Lightbend, Inc.
+ *
+ * Licensed under Apache License 2.0
+ * (http://www.apache.org/licenses/LICENSE-2.0).
+ *
+ * See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.
+ */
+
+package dotty.tools.dotc.util
+
+import collection.mutable, mutable.ListBuffer
+import scala.util.chaining.given
+import java.lang.System.lineSeparator
+
+object StackTraceOps:
+
+  extension (t: Throwable):
+
+    /** Format a stack trace, taking the prefix span satisfying a predicate.
+     *
+     *  The format is similar to the typical case described in the Javadoc
+     *  for [[java.lang.Throwable#printStackTrace()*]].
+     *  If a stack trace is truncated, it will be followed by a line of the form
+     *  `... 3 elided`, by analogy to the lines `... 3 more` which indicate
+     *  shared stack trace segments.
+     *  @param e the exception
+     *  @param p the predicate to select the prefix
+     */
+    def formatStackTracePrefix(p: StackTraceElement => Boolean): String =
+
+      type TraceRelation = String
+      val Self       = new TraceRelation("")
+      val CausedBy   = new TraceRelation("Caused by: ")
+      val Suppressed = new TraceRelation("Suppressed: ")
+
+      def header(e: Throwable): String =
+        def because = e.getCause   match { case null => null    ; case c => header(c) }
+        def msg     = e.getMessage match { case null => because ; case s => s         }
+        def txt     = msg          match { case null => ""      ; case s => s": $s"   }
+        s"${e.getClass.getName}$txt"
+
+      val seen = mutable.Set.empty[Throwable]
+      def unseen(e: Throwable): Boolean = (e != null && !seen(e)).tap(if _ then seen += e)
+
+      val lines = ListBuffer.empty[String]
+
+      // format the stack trace, skipping the shared trace
+      def print(e: Throwable, r: TraceRelation, share: Array[StackTraceElement], indents: Int): Unit = if unseen(e) then
+        val trace  = e.getStackTrace
+        val frames = if share.isEmpty then trace else
+          val spare   = share.reverseIterator
+          val trimmed = trace.reverse.dropWhile(spare.hasNext && spare.next() == _)
+          trimmed.reverse
+        val prefix = frames.takeWhile(p)
+        val margin = "  " * indents
+        lines += s"${margin}${r}${header(e)}"
+        prefix.foreach(frame => lines += s"$margin  at $frame")
+
+        val traceFramesLenDiff  = trace.length - frames.length
+        val framesPrefixLenDiff = frames.length - prefix.length
+        if traceFramesLenDiff > 0 then
+          if framesPrefixLenDiff > 0 then lines += s"$margin  ... $framesPrefixLenDiff elided and $traceFramesLenDiff more"
+          else lines += s"$margin  ... $traceFramesLenDiff more"
+        else if framesPrefixLenDiff > 0 then lines += s"$margin  ... $framesPrefixLenDiff elided"
+
+        print(e.getCause, CausedBy, trace, indents)
+        e.getSuppressed.foreach(print(_, Suppressed, frames, indents + 1))
+      end print
+
+      print(t, Self, share = Array.empty, indents = 0)
+      lines.mkString(lineSeparator)
+    end formatStackTracePrefix

compiler/test/dotty/tools/dotc/util/StackTraceTest.scala

@@ -0,0 +1,76 @@
+
+package dotty.tools.dotc.util
+
+import scala.util.{Failure, Success, Try}
+import scala.util.chaining.given
+
+import org.junit.Assert.{assertEquals, assertTrue}
+import org.junit.Test
+
+class StackTraceTest:
+  val CausedBy   = "Caused by: "
+  val Suppressed = "Suppressed: "
+
+  // throws
+  def sample = throw new RuntimeException("Point of failure")
+  def sampler: String = sample
+
+  // repackage with message
+  def resample: String = try sample catch case e: Throwable => throw new RuntimeException("resample", e)
+  def resampler: String = resample
+
+  // simple wrapper
+  def wrapper: String = try sample catch case e: Throwable => throw new RuntimeException(e)
+  // another onion skin
+  def rewrapper: String = try wrapper catch case e: Throwable => throw new RuntimeException(e)
+  def rewrapperer: String = rewrapper
+
+  // circular cause
+  def insane: String = try sample catch case e: Throwable => throw new RuntimeException(e).tap(e.initCause)
+  def insaner: String = insane
+
+  def repressed: String = try sample catch case e: Throwable => throw new RuntimeException("My problem").tap(_.addSuppressed(e))
+  def represser: String = repressed
+
+  // evaluating s should throw, p trims stack trace, t is the test of resulting trace string
+  def probe(s: => String)(p: StackTraceElement => Boolean)(t: String => Unit): Unit =
+    import StackTraceOps.formatStackTracePrefix
+    Try(s).recover { case e => e.formatStackTracePrefix(p) } match
+      case Success(s) => t(s)
+      case Failure(e) => throw e
+
+  @Test def showsAllTrace() =
+    probe(sampler)(_ => true)(s => assertTrue(s.linesIterator.length > 5))
+
+  // summary + one frame + elision
+  @Test def showsOnlyPrefix() =
+    probe(sample)(_.getMethodName == "sample")(s => assertEquals(3, s.linesIterator.length))
+
+  // summary + one frame + elision, caused by + one frame + elision
+  @Test def showsCause() = probe(resampler)(_.getMethodName != "resampler") { s =>
+    val res = s.linesIterator.toList
+    assertEquals(6, res.length)
+    assertTrue(res.exists(_.startsWith(CausedBy)))
+  }
+
+  // summary + one frame + elision times three
+  @Test def showsWrappedExceptions() = probe(rewrapperer)(_.getMethodName != "rewrapperer") { s =>
+    val res = s.linesIterator.toList
+    assertEquals(9, res.length)
+    assertTrue(res.exists(_.startsWith(CausedBy)))
+    assertEquals(2, res.collect { case s if s.startsWith(CausedBy) => s }.size)
+  }
+
+  // summary + one frame + elision times two with extra frame
+  @Test def dontBlowOnCycle() = probe(insaner)(_.getMethodName != "insaner") { s =>
+    val res = s.linesIterator.toList
+    assertEquals(6, res.length)
+    assertTrue(res.exists(_.startsWith(CausedBy)))
+  }
+
+  @Test def showsSuppressed() = probe(represser)(_.getMethodName != "represser") { s =>
+    val res = s.linesIterator.toList
+    assertEquals(6, res.length)
+    assertTrue(res.exists(_.trim.startsWith(Suppressed)))
+  }
+end StackTraceTest